1. Field of the Invention
The present invention relates to an echo canceller used to eliminate an echo generated due to acoustic coupling between a telephone talker and a listener in a hand-free telephone system and the like.
2. Description of the Related Art
In a car phone system, for example, a hand-free telephone system is expected to be useful for ensuring operability and safety of a driver. The hand-free telephone system is a system to speak by using a microphone (transmitter) and a loudspeaker (receiver) located in advance at a specified position in a car.
In the hand-free telephone system, speech quality is reduced due to a mixed noise between a telephone talker and a listener. The noise includes (1) an echo noise generated by acoustic coupling between a microphone (transmitter) and a loudspeaker (receiver) and (2) an ambient noise caused by ambient sounds, running sounds and the like. A sending signal emitted by the microphone (transmitter) is associated, in real time, with a receiving signal received by a loudspeaker (receiver). If the echo noise is mixed with the sending signal, the echo noise can be easily detected by referencing to the receiving signal itself being a noise source. Based on such detecting results, a pseudo noise is generated and subtracted from the sending signal. Such principles have been employed in a conventional echo canceller.
However, such conventional technology has problems to be solved as described below. That is, in the conventional echo canceller, a noise is detected by comparing a sending signal with a noise mixed with a receiving signal being a noise source. Accordingly, if a noise not caused by the receiving signal, i.e., an ambient noise described above as a type of the noise (2) is mixed with a sending signal, accurate detection of echo noises is impossible. Moreover, based on the results of such conventional detection, a pseudo noise is generated and is subtracted from the sending signal. Thus, an echo noise cannot be eliminated and it is a cause for a foreign voice (i.e., noise).
In view of the above, it is an object of the present invention to provide an echo canceller wherein, even during generation of ambient noises, an echo noise can be eliminated and generation of a foreign voice (i.e., noise) can be avoided, thus improving the speech quality of a hand-free telephone system.
It is another object of the present invention to provide an echo canceller wherein the detection of ambient noises are made easy and the avoidance of the generation of noises is made reliable.
According to an aspect of the present invention, there is provided an echo canceller for preventing an echo of an output from a receiver receiving a signal transmitted by a transmitter from returning to the receiver through the transmitter comprising:
a noise detecting section detecting an ambient noise surrounding the transmitter and sending out a noise detecting signal if a level of a signal showing the ambient noise exceed a predetermined value; and
an adaptive filter receiving, as an input signal, each of a receiving signal to be received by the transmitter and of a sending signal to be outputted from the transmitter and generating a pseudo echo corresponding to a change of both input signals in order to prevent the echo from returning and further operating, when receiving the noise detecting signal, to make output characteristics of the pseudo echo fixed to inhibit a change of the pseudo echo caused by the ambient noise.
In the foregoing, a preferable mode is one wherein the adaptive filter has a plurality of tap coefficients used to set a different time delay and attenuation coefficient depending on both input signals in order to generate a pseudo echo corresponding to both input signals and stops operations for changing the tap coefficient when receiving the noise detecting signal.
Also, a preferable mode is one wherein the adaptive filter is a transversal filter.
Also, a preferable mode is one wherein the noise detecting section is provided with memory to preset information about a signal level and frequency of the ambient noise.
Also, a preferable mode is one wherein the noise detecting section detects a noise signal picked up by the transmitter.
Furthermore, a preferable mode is one wherein the noise detecting section receives a noise signal picked up by a microphone designed specifically for the detection of an ambient noise and being different from the transmitter.